The invention relates generally to tomographic imaging and, more particularly, to methods and systems for correcting for motion-induced attenuation mismatch for use in tomographic imaging.
Tomographic imaging has become an integral part of healthcare services. Examples of tomographic imaging include Positron Emission Tomography (“PET”) imaging, Single Photon Emission Computed Tomography (“SPECT”) imaging, X-ray Computed Tomography (“CT”) imaging, and magnetic resonance imaging (“MRI”). In CT imaging, X-rays are propagated through the body and are detected on the other side of the body. The X-rays are attenuated to different degrees depending on encountered bodily structures, resulting in an image showing the structural features of the body. MRI also creates images of the internal structures of the human body by exploiting the property of nuclear magnetic resonance. MRI provides good contrast resolution by using various optimized radio frequency pulse sequences. Note, however, that CT imaging or MRI is not particularly sensitive to biological processes and functions.
On the other hand, PET or SPECT imaging produces images of various biological processes and functions. In PET imaging, a solution including a tracer is injected into a subject or patient to be scanned. The tracer is a pharmaceutical compound including a radioisotope with a relatively short half-life, such as 18F-Fluoro-2-Deoxyglucose (“FDG”), which is a type of sugar that includes radioactive fluorine. The tracer can be adapted such that it is attracted to sites such as lesions within the subject where specific biological or biochemical processes occur. The tracer moves to and is typically taken up in one or more organs of the subject in which these biological and biochemical processes occur. For example, cancer cells may metabolize the tracer, allowing the PET scanner to create an image illuminating the cancerous region. When the radioisotope decays, it emits a positron, which travels a short distance before annihilating with an electron. The short distance, also referred to as the positron range, is typically of the order of 1 mm for FDG in common subjects. The annihilation produces two high-energy photons propagating in substantially opposite directions.
PET imaging uses a photon detector array arranged around a scanning area, usually in a ring-shaped pattern, in which the subject or at least the part of interest of the subject is arranged. When the detector array detects two photons within a short timing window, a so-called “coincidence” is recorded. The line connecting the two detectors that received the photons is called the Line Of Response (“LOR”). The reconstruction of the image is based on the premise that the decayed radioisotope is located somewhere on the LOR. The relatively short positron range may be neglected or may be compensated for in the reconstruction. Each coincidence may be recorded in a list by three entries: two entries representing the two detectors and one entry representing the time of detection. The coincidences in the list may be grouped in one or more sinograms. A sinogram is typically processed using image reconstruction algorithms to obtain volumetric medical images of the subject. However, PET imaging does not generally provide structural details as well as other types of scanners such as CT and MRI scanners.
A PET-CT scanner includes both a CT scanner and a PET scanner installed around a single patient bore. A PET-CT scanner creates a fused image including a PET image spatially registered to a CT image. PET-CT scanners provide the advantage that the functional and biological features shown by the PET scan may be precisely located with respect to the structure illuminated by the CT scan. In a typical PET-CT scan, the patient first undergoes a CT scan, and then the patient undergoes a PET scan before exiting the scanner. After the CT and PET data have been acquired, the PET-CT scanner processes the data and generates a fused PET-CT image.
Patient motion, such as motion due to respiration, may be a significant factor in degrading the quantitative integrity of PET images. Respiratory motion may result in artifacts and/or contrast dilution of lesions from motion blurring. Respiratory-gated acquisition of PET data can reduce motion blur. In a respiratory-gated acquisition, the data is partitioned during each respiratory cycle to produce independent PET images for each partition or gate. Each of these PET images may have reduced motion blur compared to the un-gated image. However, the reduction in blur may come at the expense of increased image noise because each gate has fewer counts than the un-gated image. A PET image, nearly free of respiratory motion, with an improved signal to noise ratio may be generated by an RRA (Reconstruct, Register and Average) procedure wherein the PET images reconstructed independently for each gate are registered and then averaged.
For accurate PET quantitation, it is important to correct for both photon attenuation and respiratory motion. Although the RRA approach using respiratory gated PET data may address respiratory motion blur, a respiratory phase mismatch may exist between an attenuation image that is used in a PET image reconstruction process and each respiratory gated PET data set since the attenuation image may be obtained from a CT image, which is usually taken while the subject is holding his or her breath. This mismatch may generate undesirable artifacts in reconstructed PET images for each gate or the output PET image of the RRA approach, preventing accurate PET quantitation.
It would therefore be desirable to enhance quantitative accuracy in tomographic imaging by providing both attenuation and motion correction in an accurate manner.